1. Field of the Invention
The invention relates to an apparatus for confining laser beams of medium and high power with a diaphragm confining the laser beam.
2. Description of Related Art
For a great variety of reasons it is frequently necessary to geometrically confine or restrict laser beams. For example, it may be necessary to adapt the dimensions of the laser beam to a given optical system, generate a beam with a specifically configured cross-section, produce sharp beam boundaries or reduce the beam opening angle.
In the prior art for confining the laser beam diaphragms are usually employed which absorb the unrequired parts of the laser beam. The diaphragms may have various forms depending on the desired dimensions of the confined or limited laser beam. Usually, circular pinhole or rectangular slit diaphragms are employed, depending on whether the dimension of the laser beam is to be limited in one or two dimensions. Diaphragms may be arranged in the laser beam outside the laser (extracavity) or in the laser resonator (intracavity).
The known diaphragm arrangements involve however problems when the power of the laser lies in the medium or higher range, i.e. is a few Watts or more. Since the known diaphragms limit the beam by absorption of beam components the diaphragms can become very hot and assume temperatures of more than 100.degree. C. This heating of the diaphragms leads to a heating of the surrounding air and as a result schliere formation can occur at the edges. The density of the air changes directly at the edges of the diaphragm so that inhomogeneity of the refractive index occurs in said region and this causes the schliere formation.
Such schliere formation or streaks disturb the rectilinear propagation of the laser beam. The schliere formation impairs the quality of the laser beam and under unfavourable conditions may even partially cancel the effect of one or more diaphragms which are intended to sharply restrict the beam or confine the beam opening angle. Due to the schliere formation the edge parts of the laser beam are irregularly diffracted so that the sharp edge boundaries become blurred again and parts of the beam can be reflected into precisely those directions which the diaphragms are intended to block off.